Particles, each having a metal surface, have been used as various resin fillers, modifying agents and the like, and in addition to these usages, those particles are mixed in a binder resin as conductive particles, and used as so-called anisotropic conductive materials that electrically connect miniature-size electrical parts such as semiconductor elements to circuit boards, or electrically connect circuit boards to each other, in electronic products such as liquid crystal displays, personal computers and portable communication devices.
In recent years, along with the developments of miniature-size electronic apparatuses and electronic parts, wires on circuit boards and the like have become finer, and under these circumstances, conductive particles have been made further finer, and the precision of particle sizes has been improved. In order to ensure high connection reliability, it is necessary to increase the blended amount of conductive particles in an anisotropic conductive material, and in the case of a circuit board and the like having such fine wires, conduction or the like in a lateral direction may occur between adjacent conductive particles to cause problems of short circuit and the like between adjacent electrodes. In order to solve these problems, an anisotropic conductive material, which uses conductive particles of which surfaces are coated with an electrical insulating material, has been proposed.
With respect to the method for coating the surfaces of conductive particles with an electrical insulating material, for example, Japanese Kokai Publication Hei-4-362104 has disclosed a method in which an interface polymerizing process, a suspension polymerizing process, an emulsion polymerizing process or the like is carried out in the presence of conductive particles so that the particles are encapsulated in an electrical insulating resin, Japanese Kokai Publication Sho-62-40183 has disclosed a method for forming microcapsules with an electrical insulating resin by a dipping process in which conductive particles have been dispersed in the resin solution, and then dried, and Japanese Kokai Publication Hei-7-105716 has disclosed methods in which a hybridization process are used for this purpose; and in addition to these methods, methods using vacuum vapor deposition and the like have been known.
In these methods, however, it is difficult to form an insulating coating layer with a constant thickness, and in some cases, a plurality of conductive particles are simultaneously coated. In the case where conductive connection is made by using coated conductive particles, if the thickness of the insulating coating layer is not uniform, a pressure is not transmitted uniformly upon fixing the layer between electrodes even when the particle size of the conductive particles is precisely controlled, with the result that a defective conduction may occur. For example, in the case of the above-mentioned formation method of the insulating coat by the hybridization process, since insulating resin particles to form a coating layer are made to adhere to the surfaces of the conductive particles by a physical force in this method, it is not possible to form the coating layer on the surface of each of the conductive particles as a single layer, with the result that it becomes difficult to control the thickness of the insulating coating layer, and since the resin particles are fused and deformed due to heat and impact caused by the heating process and frictional heat, it is difficult to prepare an uniform coating layer. Moreover, since the contact area between the insulating resin particles and the metal surface becomes greater, it is difficult to remove the insulating coating layer in the case where a device to which it is difficult to apply heat and pressure, such as a liquid crystal element, is used, with the result that a defective conduction may occur.
Japanese Kokai Publication Hei-4-259766 and Japanese Kokai Publication Hei-3-112011 have disclosed coated conductive particles in which insulating particles are adhered weakly to the surfaces of conductive particles by an electrostatic interaction and a hybridization method. However, in the coated conductive particles obtained by these methods, since a bonding force between the insulating particles and the conductive particles, which is dependent only on Van der Waals force or electrostatic force, is very weak, insulating particles are separated from the conductive particles by dispersion in a binder resin and contact between adjacent particles. As a result, failing to ensure a sufficient insulating property occur.
Moreover, conventionally, upon forming an anisotropic conductive material by dispersing such coated conductive particles in a binder resin, those coated conductive particles having a coating layer that is non-compatible to the binder resin, solvent and the like have been used. For example, Japanese Kokai Publication Hei-4-362104 has disclosed a polymer coating method for metal particles in which a homopolymer layer or a copolymer layer that is non-compatible to a binder resin is formed on the surface of metal particles; Japanese Kokai Publication Sho-62-40183 has disclosed an electrical connecting sheet which is formed by dispersing conductive particles in a hot-melt type insulating adhesive, and is characterized in that the conductive particles are coated with a resin that is non-compatible to the hot-melt type insulating adhesive; and Japanese Kokai Publication Hei-7-105716 has disclosed coated conductive particles each of which is composed of an insulating core material, a conductive layer formed on the core material and an insulating layer that covers 0.1 to 99.9% of the area of the conductive layer.
However, when the conductive particles each of which has a coating layer that is non-compatible to a binder resin are used, affinity in the interface between the binder resin and the coated conductive particles becomes poor, with the result that the coated conductive particles dispersed in the binder resin may cause a phase separation and the like and the resulting poor connecting stability. In particular, in the case of an anisotropic conductive film and an anisotropic conductive adhesives using thermosetting resin as a binder resin, since the affinity in the interface between the binder resin and the coated conductive particles is poor, a separation occurs in the interface between the binder resin and the coated conductive particles after the binder resin has been cured by thermocompression bonding, failing to ensure long-term stability and reliability in connection. Moreover, in the case where coated conductive particles are dispersed in the binder resin such as a sealing agent or the like in order to maintain a gap between the electrodes as well as between liquid crystal panels, since the resin used for forming the coating layer is non-compatible to the binder resin, there is a problem that coated resin thermally fused may bleed out to pollute electrodes, liquid crystal and the like.